The invention concerns correcting vignetting in an imaging chain for processing image data obtained from sensors of a digital imaging device into a processed image.
Vignetting is a well-known effect observable especially in digitally recorded images and characterized as a circularly symmetrical shading in the border areas of the image. In other words brightness of the image decreases towards borders of the image. The vignetting is sometimes called a light fall-off. Part of the vignetting effect is due to the imaging optical components, e.g. lenses, since the refraction in the border areas of the optics typically cause decrease in light level of the final image near the image borders. Part of the vignetting effect is sensor related. Factors effecting sensor related vignetting are micro lenses and the fill factor of the sensors. The fill factor is a ratio of the light sensitive area of a sensor pixel on its total area. Fill factors of pixels are improved by applying in front of each pixel a tiny individual micro lens that converges the light hitting the micro lens further on the even smaller-sized light sensitive element, e.g. photodiode, below it. Further, if the incoming light hits the sensor in deep enough angles, it may be absorbed to the “pixel walls” instead of reaching the actual light sensitive element on the bottom of the sensor.
1. Description Of The Prior Art
Some digital camera manufacturers do not correct the vignetting effect at all. With high quality optics vignetting may be neglible and it does not need to be corrected. However, typically digital camera sensor manufacturers use some sort of vignetting elimination. Especially in the very small sized digital camera modules, which typically include optical modules integrating both the optics and the sensors, there usually is a vignetting effect present. Usually the vignetting effect in these small and low-cost devices is significant and some correction for vignetting is required.
In the prior art solutions the vignetting effect is commonly corrected by compensating brightness of the image after the image has been recorded. Typically only the luminance level of the final image is corrected. In many cases a more accurate, but still simple method is needed in order to achieve a better image by a small and compact imaging device. Typically in prior art solutions either the correction is demanding, hard and time-consuming to implement, or the correction is only an average resulting in an inaccurate final image. One disadvantage with the prior brightness correction is that the colors having different wavelengths have been treated in a similar manner. Typically, correction values used in the prior art are average values and no chromatic correction is applied, although sensors include pixels with different colors and also the final image is presented in colors.
2. Brief Description Of The Invention
The aim of the present invention is to overcome the drawbacks of the prior solutions for correcting a vignetting effect in the final processed image. One aim of the present invention is to enhance quality of the final image especially by compensating for a vignetting effect more accurately and separately for all primary colors of the sensor. Another aim of the present invention is to keep the solution simple and usable especially in compact, small, cheap and lightweight camera devices. A further aim of the invention is to provide a simple and effective way to characterize the vignetting related performance of an imaging module using only a limited number of parameters.
The aim of the present invention is achieved by determining and compensating for the vignetting effect of the digital image captured with the imaging module separately for at least two of the primary color components of the imaging module.
A method according to the invention for forming a model for improving image quality of a digital image captured with an imaging module that comprises at least an imaging optics and an image sensor, where the image is formed through the imaging optics onto the light sensitive image sensor, is characterized in that                determining a vignetting effect of the digital image captured with the imaging module separately for at least two of the primary color components of the imaging module,        formulating vignetting models for at least two of the primary color components of said imaging module, and        combining said vignetting models into a single correction function defined using correction coefficients, the correction function defining a correction gain for different points (x,y) of the image.        
A model according to the invention for correcting vignetting from a digital image captured with an imaging module that comprised at least an imaging optics and an image sensor, where the image is formed through the imaging optics onto the light sensitive image sensor, is characterized in that the model is obtainable by said method according to the invention for forming a model for improving image quality of a digital image captured with an imaging module.
A model according to the invention can be used for correcting vignetting from a digital image.
A method according to the invention for improving image quality of a digital image captured with an imaging module including an imaging optics and an image sensor, where the image is formed through the imaging optics onto the light sensitive image sensor, is characterized in that                a vignetting effect is separately compensated for at least two of primary colors of the imaging module by applying a correction function common for at least two of the primary color components of said imaging module, the correction function including correction coefficients pre-determined for a certain imaging module and a pre-determined correction gain for different points (x,y) of the image.        
Apparatus according to the invention for determining a model for improving image quality of a digital image captured with an imaging module including an imaging optics and an image sensor, where the image is formed through the imaging optics onto the light sensitive image sensor, is characterized in that the apparatus includes:                means for determining a vignetting effect separately for at least two of the primary colors of the image module,        means for formulating vignetting models for at least two of the primary color components of said imaging module, and        means for combining said vignetting models into a single correction function defined using correction coefficients, the correction function including means for defining a correction gain for different points (x,y) of the image.        
An imaging module according to the invention and including an imaging optics and an image sensor for forming an image through the imaging optics onto the light sensitive image sensor is characterized in that a model of the kind described above according to the invention for correcting vignetting from a digital image captured with an imaging module is related to said imaging module.
A device according to the invention is characterized in that the device includes an imaging module of the kind described above according to the invention.
A program module according to the invention for compensating a vignetting effect in a device including an imaging module is characterized in that the program module includes programmable means for implementing a method of the kind described above according to the invention for improving image quality of a digital image captured with an imaging module.
A program product according to the invention for compensating of a vignetting effect in an imaging module is characterized in that the program product includes software means for implementing a method of the kind described above according to the invention for improving image quality of a digital image captured with an imaging module.
Embodiments of the present invention are described in detail below.
Apparatus according to embodiments for determining a model for improving image quality of a digital image captured with an imaging module including an imaging optics and an image sensor, where the image is formed through the imaging optics onto the light sensitive image sensor, includes means for determining a vignetting effect separately for at least two of the primary colors of the image module.
According to an embodiment apparatus includes means for formulating vignetting models for at least two of the primary color components of said imaging module, and means for combining said vignetting models into a single correction function defined using correction coefficients, the correction function including means for defining a correction gain for different points (x,y) of the image.
According to embodiments a method for forming a model for improving image quality of a digital image captured with an imaging module including an imaging optics and an image sensor, where the image is formed through the imaging optics onto the light sensitive image sensor, includes a step of determining a vignetting effect of the digital image captured with the imaging module separately for at least two of the primary color components of the imaging module. According to embodiments the method includes the steps of formulating vignetting models for at least two of the primary color components of said imaging module, and combining said vignetting models into a single correction function defined using correction coefficients, the correction function defining a correction gain for different points (x,y) of the image.
According to an embodiment of the present invention the raw data achieved from the camera-sensor-module is processed so that the vignetting effect from the final processed image is corrected accurately. According to the embodiment of the present invention correction is implemented with a simple model having minimum number of parameters. In the embodiments of the present invention the good quality of the correction is achieved by a simple and light model that is executable also with small, compact devices.
According to an embodiment of the present invention the vignetting correction is modelled with a polynomial that is parameterized. According to one embodiment the polynomial is a 6th degree polynomial. In embodiments of the invention, the polynomial contains a parameter for modelling the difference in vignetting between different primary color components. Thus, in the present invention, the differences between color components are taken into account. The applied color components can be three color RGB-components (red, green, blue), four color CMYG-components (cyan, magenta, yellow, green) or any other chosen primary color components. Also according to the correction of the present embodiment it is possible to define a center for vignetting that is different from the image center. This is advantageous for slightly misaligned optics, in which the optical axis and the center axis of the visible pixels are not exactly congruent for manufacturing reasons, etc.
According to another embodiment of the present invention, the vignetting correction is modelled with a polynomial including a parameter for describing a pixel shape. In this embodiment there is also modelled pixels that are not square shaped. Some sensors have pixels having a height different than the width. Different kinds of non-square shapes of pixels are taken into account in this embodiment of the present invention. The model of the embodiment includes a parameter, which describes the shape of the pixel, which is taken into consideration also when forming the final image, i.e. when using the formed model.
The algorithm presented in the embodiments of the present invention is simple. One advantage is that the correction parameters need to be determined only once to a certain camera-sensor-type. After the determination, parameters can be used for modelling and correcting images produced with said camera-sensor-type. Further, the correction according to embodiments of the present invention is easy to transfer from sensor manufacturer to camera manufacturer, since only few parameters are needed to characterize the correction function used for the sensor module. The minimum information is required to be transmitted to the camera manufacturer and to be stored and used in a final product (i.e. camera) when using the vignetting compensation. Due to the simplicity of the method of the embodiments, it is possible, if necessary, to characterize individual optical modules during their manufacture and to implement individual correction for those modules in the final camera products.
In embodiments of the present invention the number of characterizing features in the correction function is kept to a minimum. This allows easy introduction of new sensor modules by applying only a minimum number of parameters. It is enough that the sensor manufacturer transmits with a new sensor the certain coefficients and parameters required that characterize the sensor in question. The coefficients and parameters used in the model are typically the same for a certain set of sensors. According to one embodiment, the coefficients and parameter values are sensor-specific. Typically also the optics used influences the parameters of the model. According to an embodiment, parameters and coefficients of the model are determined for a certain optic-sensor-module. If there are any changes in optics or in sensors, parameters and coefficients for the model are determined again. A new determination according to an embodiment is easy and fast to implement, so vignetting is modelled effortlessly to a new configuration of optic-sensor-module.
In the embodiments of the present invention the required parameters describe size of an image plane, vignetting effect in a certain point (x,y) of the image plane, degree of compensation and coefficients for each primary color components. Embodiments of the invention allow the manufacturer of a module having imaging optics and imaging light sensor to characterize the vignetting performance of the module and provide the camera manufacturer implementing the module in his product a simple set of parameters, which parameters can be used in the end product to make the necessary image correction.
According to the embodiments, the amount of information is kept to a minimum and the implementation is kept easy, simple and cheap. The parameters are easy and effortless to transmit and execution of the model remains light. According to embodiments of the present invention only a little amount of memory is consumed when modelling and compensating the vignetting effect in a camera device. Further parameters and coefficients for modelling are determined only once for a certain camera-sensor-module, so continuous calculations are not required.
According to embodiments of the present invention the model is easily applicable for different sensors by only using a different set of parameters. The modelling, i.e. use of the model with the determined parameters and coefficients, according to embodiments is executable also in small, compact devices effectively. Embodiments can be used with zooming and cropping functions. One important benefit of embodiments is that scaling of an image does not change the parameters used in the model.